1. Field of the Invention
This invention relates generally to a device for modulating a clutch pressure and more specifically to a variable orifice device for modulating the clutch pressure applied to a fluid pressure operated clutch of a power transmission of an industrial vehicle, for example, a fork lift.
2. Description of the Prior Art
In a conventional clutch pressure modulator valve device, the valve device comprises a piston disposed in an accumulator chamber for accumulating oil pressure for operating a clutch of a power transmission and a relief port for transmitting a portion of the pressurized oil in the accumulator chamber to an oil reservoir upon movement of the piston. In this way, the oil pressure operating clutch in the accumulator chamber increases along a gradual pressure gradient to a predetermined value. Subsequently, communication between the accumulator chamber and the relief port is cut off by further movement of the piston after the oil pressure in the accumulator chamber applied for operating the clutch is increased to the predetermined value. In this type of valve, however, the effective diameter of the orifice in the relief port for determining the pressure gradient of the modulating characteristic of the oil pressure is fixed and cannot be controlled from the exterior of the power transmission.
It is desirable that the modulating characteristic of the pressure gradient of the modulator valve changes in response to an operating condition or load condition of the vehicle in order to operate the clutch under an optimum pressure gradient by changing any one of several operating parameters. For example, a spring force biasing the piston, the volume of the accumulator chamber or the flow rate of oil into the accumulator chamber may be changed. As a result of this change, a shock can be generated due to the sudden engagement of the clutch when the vehicle is under a light load due to a very steep pressure gradient of the modulating characteristic or an increase in time lag of clutch operation can occur when the vehicle is under a heavy load due to a very gradual pressure gradient of the modulating characteristic.